A fusion sealing material may be applied to a single sealing surface, as in glazing or enameling. Alternatively, the material may be employed to join two opposed surfaces. The result is then referred to as an intermediate seal or joint. The present invention is applicable to either type of sealing operation.
As here used, the term "fusible sealing material" connotes material that may be thermally softened to flow and wet a surface, and that normally is composed of a base glass with or without a modifying additive. A "fused seal" is the result of fusing such material on, or between, sealing surfaces.
Fusion sealing is a well cultivated art. In particular, numerous special sealing glasses have been developed for use in joining glass parts with each other, or with metals, alloys, or ceramics.
In making a fusion seal, the sealing glass must be heated to a temperature where it becomes soft enough to wet the sealing surface and form an adherent, hermetic bond. For many purposes, it is desirable to maintain the sealing temperature as low as possible. This is particularly true in electrical and electronic articles where thermally sensitive parts or coatings are commonly employed.
Accordingly, considerable attention has been given to lead glasses as low temperature sealing glasses. For example, stable sealing glasses, having softening points in the 430.degree.-500.degree. C. range, and coefficients of thermal expansion in the 70-90.times.10.sup.-7 /.degree.C. range, are disclosed in U.S. Pat. No. 2,642,633 (Dalton). Subsequent studies centered on lead zinc borate type glasses that were subject to thermal devitrification or crystallization. These glasses were intensively investigated in the search for cathode ray tube sealing materials.
Lead sealing glasses are widely used in conjunction with materials having CTEs in the 70-100.times.10.sup.-7 /.degree.C. range. These include soda lime type glasses and various ceramics and metals. However, lead glasses are ineffective for lower expansion materials, such as fused silica, borosilicate glasses, and alumina. Some degree of expansion mismatch may be tolerated in some circumstances, but must be limited, and is generally undesirable.
The sealing art has recognized that high CTE values can be lowered by mill additions of materials having lower CTE values. Numerous additives have been proposed, particularly for use with the lead borate and lead zinc borate glasses. Among these are titanates, zircon and quartz. Also, crystalline materials having negative CTEs, such as beta-eucryptite, have been proposed.
In selecting a mill addition for CTE control, various other factors must also be considered. For example, it is important that the material be relatively inert. In particular, a mill addition should not react with, or dissolve in, the glass to which it is added. Not only will the CTE control be lost or modified, but such undesirable effects as bubbling, devitrification, and/or physical property changes may occur. Some additives, especially if used in large amounts, may sharply raise viscosity. This adversely affects flow characteristics during sealing.